FIG. 1 shows a conventional aero engine fan rotor and front bearing housing arrangement.
A fan disc 100 supports a circumferential row of fan blades 102. The disc requires torsional stiffness to resist the 2D vibration mode of the fan rotor. This torsional stiffness is provided by front 104 and rear 106 axially spaced diaphragms which have differential hoop bending stiffnesses. An optional middle diaphragm 108 may be provided to produce a more even radial load distribution across the fan blade root fixings.
The disc 100 has a rear drive arm 110 which extends to a bolted joint with the engine's fan shaft 112.
A front bearing arrangement including a housing 114 and a front set of ball bearings 116 and a rear set of roller bearings (not shown), support and locate the fan shaft.
The front set of bearings 116 is axially offset a distance O rearwards from the centre of gravity C of the fan blades 102. This results in a large overhung fan mass having a relatively low first order vibration frequency with large vibrational amplitudes and high restoring forces.
Very large forces are generated during a fan blade off (FBO) event, producing a large bending moment in the fan shaft 112 due to the FBO force and the axial offset O of the front bearing. This bending moment is reacted at the bearings 116 along with the direct forces.
The rear drive arm 110 is relatively long, its length being determined by the relative fan disc 100 to bolted joint radial growth in order to provide an amount of isolation to the bolted joint. The length of the arm causes the front set of bearings 116 to be pushed further away from the fan blade centre of gravity C. Moreover, because of the large FBO loads experienced by the bearings 116, the arm 110 has to be oversized to ensure structural integrity.
The already expensive fan disc forging is thus made even more expensive by its accommodation of a long and oversized rear drive arm 110.
An alternative to a bolted joint for connecting the fan disc to the fan shaft is to use a splined joint, as proposed in GB 1556266 and GB 1215300. However, in such spline drive fan discs, the support structure of the foremost bearing and the bearing envelope interferes with the space available for the conventional fan disc rear diaphragm. Consequently, fan disc torsional stiffness is reduced, increasing the risk of coincidence of the 2D fan rotor frequency with an engine order forcing frequency. Also, during FBO, conventional front and rear disc diaphragms provide structural strength to the disc to resist hoop bending distortion and prevent potential disc burst from excessive localised hoop stress in the disc bore.